Lauryllactam is widely used for the preparation of polyamide 12. Its production on an industrial scale is well known [see for example "Precede de Petrochimie" (Petrochemical Process), Volume 2, pp. 316-322, published by Technip, 1986]. It is possible, for example, to manufacture lauryllactam from cyclododecane (HULS and Ato Chimie process), from cyclododecanone (UBE process) or alternatively from cyclododecatriene monoozonide (Snia Viscosa process).
In the process developed by Ato Chimie, the lauryllactam is obtained in two stages:
in the first stage, cyclododecanone oxime hydrochloride is formed by photonitrosation of cyclododecane solubilized in a chlorinated solvent according to the following reaction: ##STR1## PA1 and, in the second stage, the reaction product is subjected to a Beckmann rearrangement in the presence of sulphuric acid: ##STR2## PA1 colours the reaction medium, thus causing a loss in the number of photons essential for the reaction, PA1 disperses with difficulty in the reaction medium because of its high viscosity and its ionized character, PA1 reacts with nitrosyl chloride to form nitrosyl acid sulphate, which sulphate contributes towards degrading the oxime hydrochloride, PA1 hydrolyses the oxime hydrochloride to cyclododecanone and hydroxylamine, and PA1 reacts with the nitrosing agent, which has the effect of retarding the onset of the reaction and, consequently, of reducing production. PA1 in the presence of organic compounds, sulphuric acid decomposes, releasing undesirable sulphur dioxide whose content increases during recycling of the organic phase containing the unreacted cyclododecane, PA1 a portion of the oxime is hydrolysed to the corresponding ketone, PA1 a portion of the lactam is hydrolysed to the corresponding amino acid which, under the process conditions, cannot be recovered and profitably exploited, and PA1 sulphuric acid is capable of decomposing the chlorinated reaction solvent carried by the acidic phase to phosgene which is toxic for humans. PA1 a-photochemically reacting cyclododecane solubilized in an organic solvent, a nitrosing agent and hydrogen chloride in the presence of an acid in order to form cyclododecanone oxime, and PA1 b-subjecting the said oxime to a Beckmann rearrangement in the presence of an acid, the said process being characterized in that the acid used is methanesulphonic acid.
The cyclododecanone oxime hydrochloride formed during the photonitrosation is solid and very stable, and it becomes deposited on the walls of the irradiation lamps in contact with the reaction medium. Under the effect of light, the deposit is gradually converted to a tarry substance which, in the relatively long term, causes the termination of the photochemical reaction.
To overcome this difficulty, it is known to add sulphuric acid to the reaction mixture (see for example FR-B-1,335,822 and FR-B-1,553,268). In this manner, the sulphuric acid extracts the cyclododecanone oxime hydrochloride as it is formed. Having avoided any risk of deposition, it becomes possible to carry out the reaction continuously.
However, the use of sulphuric acid has disadvantages, both at the level of the first and second stage of the process.
In the photonitrosation stage, sulphuric acid:
In the Beckmann rearrangement stage, the reaction is carried out at a high temperature, greater than 135.degree. C. Consequently, the disadvantages linked to the use of sulphuric acid are as follows:
Furthermore, all the sulphuric acid-containing effluents generated by the industrial process can only be recycled at the cost of a long, difficult and costly treatment because it is in particular necessary to use steps for concentrating the acid and for removing organic compounds.